5.5.2 — Protocol command syntax error

What this code means

A mail system permanently rejected a mail transaction protocol command because it could not interpret it: the syntax was wrong or the command was unrecognized. Do not retry the same unchanged attempt.

Technical meaning

The X.5.2 pattern means that a mail transaction protocol command was issued that could not be interpreted because its syntax was wrong or the command was unrecognized. The registry description says this detail is useful only as a permanent error, and the concrete 5.5.2 variant has the corresponding class 5.

Delivery status

The leading digit 5 denotes a permanent failure for this message in the current context. The code alone identifies neither the exact command nor its faulty part, does not determine whether the cause was invalid syntax or an unrecognized command, and does not establish which side has the problem.

Class: Permanent failure
Retry: Do not retry unchanged
Suppression: Check the full context

Retry decision

Operational recommendation: stop automatic and manual retries of the same command in the unchanged context. Consider a new send only after the syntax has demonstrably been corrected, the unrecognized command has been removed or replaced, or its handling has demonstrably changed; first check suppression again and use idempotency.

Suppression decision

Operational recommendation: do not add the address to a suppression list based on 5.5.2 alone because the code describes a protocol-command interpretation problem, not mailbox status. Inspect the full response, transaction trace, other permanent events, and the applicable policy before suppressing.

Common causes

A mail transaction protocol command had invalid syntax.
The system that returned the code did not recognize the issued protocol command.

Diagnostic steps

Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 5.5.2 and that the basic reply is in the 5xx class.
Correlate the response with the intended message, attempt time, system that returned the code, and exact command; also retain the preceding transaction trace.
Compare the command with the applicable protocol syntax and inspect available logs to determine whether it was malformed or unrecognized; do not choose a cause from the code alone.
Stop unchanged retries; before any new send, confirm a relevant correction or handling change and check suppression state again.

Actions by owner

Sender administrator

Retain the full response, exact command, and preceding transaction trace, then correct any confirmed syntax defect or remove or replace the unrecognized command.
Stop retries in the unchanged context; permit a new send only after a correction or handling change has been confirmed and suppression has been checked again.

Provider

For the specified time and system, inspect logs, the protocol parser, and the transaction trace to identify the exact command and distinguish invalid syntax from an unrecognized command.
Correct any confirmed problem in the managed implementation, or tell the sender administrator the required syntax and recognized command set while preserving the exact status code in the response.

Verified provider examples

Verified provider examples for 5.5.2 appear below, from Rackspace Email. They show real provider practice for this response and do not redefine the standard 5.5.2 meaning or cover every occurrence.

Rackspace Email hosted mail · SMTP 554

Exact SMTP response

554 5.5.2 <user@nonqualifieddomain>: Invalid data in message> #SMTP#

Rackspace Email hosted mail · SMTP 504